1. Field
The present disclosure relates generally to circuits for driving light-emitting diodes (LEDs) and, more specifically, to LED driver circuits with open load detection.
2. Related Art
LED lighting has become popular in the industry due to the many advantages that this technology provides. For example, LED lamps typically have a longer lifespan, require less power, pose fewer hazards, and provide increased visual appeal when compared to other lighting technologies, such as compact fluorescent lamp (CFL) or incandescent lighting technologies. The advantages provided by LED lighting have resulted in LEDs being incorporated into a variety of lighting technologies, televisions, monitors, and other applications.
It is often desirable to implement LED lamps with a dimming functionality to provide variable light output. One known technique that has been used for analog LED dimming is phase-angle dimming, which may be implemented using either leading-edge or trailing-edge phase-control. A semiconductor switch-based circuit (e.g., TRIAC or MOSFET) is often used to perform this type of phase-angle dimming and operates by delaying the beginning of each half-cycle of alternating current (ac) power or trimming the end of each half-cycle of ac power. By delaying the beginning of each half-cycle or trimming the end of each half-cycle, the amount of power delivered to the load (e.g., the lamp) is reduced, thereby producing a dimming effect in the light output by the lamp. In most applications, inconsistences in the delay at the beginning of each half-cycle or in trimming of the end of each half-cycle are not noticeable because the resulting variations in the phase-controlled line voltage and power delivered to the lamp either occur more quickly than can be perceived by the human eye or are averaged by the naturally slow response of the lamp. For example, dimmer circuits work especially well when used to dim incandescent light bulbs since the variations in phase-angle with altered ac line voltages are averaged by the thermal time constant of the lamp. However, flicker may be noticed when dimmer circuits are used for dimming LED tamps.
Flickering in LED lamps can occur because these devices are typically driven by LED drivers having regulated power supplies that provide regulated current and voltage to the LED lamps from ac power lines. Unless the regulated power supplies that drive the LED lamps are designed to recognize and respond to the voltage signals from dimmer circuits in a desirable way, the dimmer circuits are likely to produce non-ideal results, such as limited dimming range, flickering, blinking, and/or color shifting in the LED lamps.
Difficulties arise with a TRIAC dimmer circuit, because a TRIAC is a semiconductor component that operates as a controlled ac switch. Thus, the TRIAC operates as an open switch to an ac voltage until it receives a trigger signal at a control terminal, causing the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the “holding current.” Most incandescent tamps draw more than the minimum holding current from the ac power source to enable reliable and consistent operation of a TRIAC. However, the comparably low currents drawn by LEDs from efficient power supplies may not meet the minimum holding currents required to keep the TRIAC switches conducting for the same duration in each half-cycle of the ac input voltage. As a result, the TRIAC may trigger inconsistently. In addition, due to the inrush current charging the input capacitance of the driver and because of the relatively large impedance that the LEDs present to the input line, a significant ringing may occur whenever the TRIAC turns on. This ringing may cause even more undesirable behavior as the TRIAC current may fall to zero and turn off the LED load, resulting in a flickering effect.
To address these issues in dimmer circuits, conventional LED driver designs typically rely on current drawn by a dummy load or “bleeder circuit” of the power converter to supplement the current drawn by the LEDs in order to draw a sufficient amount of current to keep the dimmer circuit conducting reliably after it is triggered. These bleeder circuits may typically include passive components and/or active components controlled by a controller or by the converter parameters in response to the load level.
During normal operation, LED drivers provide an output having a controlled current at a voltage that is fixed by the LED load. However, in the event that the LED load is disconnected from the output of conventional LED drivers, the output voltage may rise and damage the components of the driver. In addition, the dissipation in the bleeder circuit may increase above acceptable levels. The bleeder circuit is designed to help maintain the operation of the dimmer circuit and cannot dissipate the increase in output voltage when the LED load becomes disconnected. Thus, it may be desirable to detect load disconnections and open load conditions in LED drivers.